Typical ball screw actuators employ a screw and nut of the general character disclosed in the present assignee's U.S. Pat. No. 5,485,760, for example, wherein the nut has an external ball return tube for a recirculating train of load bearing balls which is accommodated within a nut and screw groove helical raceway.
In this patent, the nut, which is fixed to a non-rotatable piston, is held against rotation. Reciprocating movement of the piston is caused by rotary movement of the screw in the usual manner via the train of load bearing balls. In another embodiment of the invention disclosed in the aforementioned patent, the screw may be revolved by a suitable motor or source of driving rotation while being fixed against axial movement and the nut, which is prevented from revolving, will be moved axially.
In the case of the configurations of the aforementioned patent, the ball screw lead may only be as small as the ball size permits. For example, if a 60 thousandths inch lead was specified, the ball diameter size could only be a fraction (for example 0.078215 inches) of this lead and a great many balls would be required for large loads. The present differential ball screw assembly permits the use of large load bearing balls (for example 0.3125 and 0.28215 inches), while providing a very fine resultant lead.